Through a modified immunohistochemical approach intracellular prolactin binding sites (IPBS) have been observed in epithelial cells of normal male rat sex accessory organs (prostate gland and seminal vesicle) and in neoplastic cells of R3327 rat prostatic carcinomas. In ventral prostate gland IPBS disappear after orchidectomy and return with androgen treatment. In other tissues androgen effects have not been examined. The immunohistochemical demonstration of IPBS involved exposing tissue sections to exogenous prolactin where the cell membrane posed to barrier and internal organelles were exteriorized. Therefore, it is not known whether IPBS represent in situ prolactin receptors (implying hormone entry) or internally manufactured receptors destined to reach the plasmalemma (implying hormone non-entry). The crucial questions of this project are whether IBPS of normal and neoplastic prostatic tissue represent in vivo prolactin receptors and whether prolactin binding in vivo is regulated by androgen. We hope to answer these questions by the following approaches: 1) characterization of androgen regulation of IPBS in ventral and dorsolateral prostate glands by studying the histological patterns and time-course of IPBS disappearance following orchidectomy and reappearance with partial, complete, and excessive androgen replacement; 2) determination of whether endogenous prolactin becomes immunohistochemically demonstrable inside or in the surface of epithelial cells of ventral and dorsolateral prostate gland when serum prolactin levels are elevated by such procedures as ether, perphenazine, or pituitary homotransplantation; 3) determination of the patterns of prolactin binding in vivo when serum prolactin is elevated (as in 2) and when IPBS have been modified by androgen withdrawal and replacement (as in 1); 4) determination of whether IPBS which we have observed in R3327 prostatic carcinomas are also in vivo prolactin receptors subject to androgen regulation.